1. Field of the Invention
The present invention relates to a developing unit for use in an image forming apparatus such as an electronic photocopying machine, a printer or the like and, in particular, to an improved a rotary-type developing unit in which a plurality of developing devices are mounted on a rotary holder and, in each of the developing devices, there is used a developer of two components consisting of a given color toner and a carrier for such toner.
2. Description of the Related Art
In recent years, in a copying machine and a printer, there has been increasing a demand for employment of color images.
In an image forming apparatus such as a copying machine, a printer or the like, to obtain color images, there is generally employed a method in which there are installed a plurality of developing devices for developing a plurality of color toners and the plurality of color toners are superimposed on top of another on a record medium.
As means for realizing the color images, there has been put to practical use a so-called rotary type of unit (that is, a rotary-type developing unit) in which developing devices for developing necessary colors are so disposed as to be opposed to photo conductors.
Further, in each of the developing devices, in most cases, there is employed a developing system which uses a two-component developer consisting of a non-magnetic toner and a magnetic carrier. In this developing system, a newly supplied non-magnetic toner must be uniformly mixed with a carrier to be thereby charged electrically to a sufficient degree for development, before the non-magnetic toner reaches a developing area where the non-magnetic toner cooperates with a latent image carrier for development of a latent image carried by the latent image carrier.
In a conventional developing device of a two-component system, in most cases, there has been employed such a method as follows: that is, there is prepared a developing housing which stores therein two-component developer and includes a developing opening formed in part thereof; a developing roller for carrying the two-component developer thereon is so disposed as to face the developing opening of the developing housing; a developer amount restrict member for restricting the amount of the two-component developer carried on the developing roller is disposed around the developing roller; a developer circulating passage used to circulate and deliver the developer along the axial direction of the developing roller is formed on the back surface side of the developing roller of the developing housing; in the developer circulating passage, the developer is circulated while it is being stirred well by two developer delivery members (i.e., augers) which can be driven or rotated in the mutually opposing directions; a toner supply port is formed in a position near the delivery start point of the deeper-side developer delivery member that is spaced apart furthest from the developing roller; in the portion of the developer circulating passage where the developer is attached to a latent image carrier such as a photo conductor or the like, while stirring and delivering the developer already existing in the developing housing with a new toner that is supplied from the toner supply port, the density of the toners are made uniform and the toners are electrically charged; the thus mixed developers and toners are finally turned into layers with the developer amount restrict member; and, after then, development is executed in the developing area.
In the above-mentioned conventional rotary-type developing unit, when the plurality of developing devices respectively use the developers different from each other to thereby form full-color images, the developing devices must be switched over to one another sequentially during the image forming operation. However, in fact, short or poor stirring and mixing of the developers is liable to occur in the respective developing devices, so that the toners cannot be mixed or charged uniformly to a sufficient degree. As a result of this, the uneven toner density and fogging in the background occur easily.
To solve the above technical problems, a conventional developing unit is known in which a guide member is provided near the developer layer restrict member. The excessive developers restricted by the developer layer restrict member are stored on the guide member. When the developing devices are sequentially moved by the rotation of the rotary holder, the stored developers on the guide member are discharged into the developers in the developing housing, thereby making up for the inadequate stirring and mixing of the developers described above (see Japanese Patent Publication No. 7-333978 of Heisei and the like).
With use of this type of developing unit, since the developers are stirred and mixed in the respective developing devices even when the rotary holder is rotated, the above-mentioned various technical problems due to the inadequate stirring and mixing of the developers may be solved.
By the way, when the present inventors have studied the relation between the developer weight (g) of the developers existing in the respective developing devices and the developed density difference (IN/OUT) of the output image after 10 sheets of images and 20 sheets of images have been formed, for example, in a 4-color mode (that is, a full-color mode), there are obtained such results as shown in FIG. 8. From these results, it is found that, when the developer weight (g) of the respective developing devices is increased, the developed density difference (IN/OUT) is decreased, so that the density stability can be enhanced. That is, based on this knowledge, the present inventors have conducted an elaborate examination on a demand for increasing the developer weight (g) of the respective developing devices as much as possible.
Now, in the above-mentioned type of developing unit, when a rotary-type developing unit must be made compact, the sizes of the respective developing devices to be incorporated into the rotary-type developing unit are limited and, as an inevitable consequence of this, the developer amount itself contained in the developing devices is limited greatly. Therefore, it is impossible to increase the developer weight and thus the life of the developer is liable to be short. Also, there is found a new technical problem that the developing unit of this type is complicated in structure.